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Dick Byrne walks his Bernese mountain dog, Josie, early every morning, come rain, shine or, in this case, icy cold. "She loves it. In fact, she doesn't want to come back inside," says Mr. Byrne, a Southeast resident and native Minnesotan. "I don't mind the cold, either, but I don't want it to last more than three, four or maybe five days." He's not the only one wishing for warmer weather. It has been on everyone's bluish lips of late. Fortunately, weather forecasters predict that this frigid week will draw to a warming close. But why did this cold spell arrive to begin with? Is naughty El Nino to blame again? "No; even if El Nino is good to blame for everything, it seems, we can't blame this cold outbreak on El Nino," says Mike Halpert, head of forecast operations at the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs. "This cold spell is related to the polar jet stream bringing arctic air out of Canada," Mr. Halpert says. Jet streams are currents of fast winds at high altitudes that generally flow from west to east, separating cold polar air to their north from warmer air to their south, says Gerry Bell, a meteorologist also with NOAA's Climate Prediction Center. During major cold-air outbreaks, the polar jet stream in Canada heads south, bringing cold temperatures with it. That doesn't mean El Nino, the periodic large-scale warming of the tropical Pacific Ocean and the global weather repercussions that accompany it, has abandoned us this season, Mr. Bell says. It is all around us because we're in an El Nino period which lasts anywhere from a year to four years. It's just not very noticeable right now. Ultimately on the whole for the year our weather is a combination of both the jet stream and El Nino."They are competing factors; overall for the winter, there's been about a 50-50 match between El Nino and the jet stream," Mr. Bell says.

The cold air is originally from the Arctic, even Siberia, and is alternately referred to as the "Arctic Express" or the "Siberian Express." In the past 20 years, this mid-Atlantic area hasn't had very many cold snaps. This is because the North Atlantic Oscillation periodic variations of the polar jet stream has kept the jet stream on a fairly straight west-to-east route. This shields the South, and even parts of the mid-Atlantic, from chilling arctic winds. "We've been in a warm phase of the North Atlantic Oscillations, which means that our recent winters generally have been milder than we remember as kids," Mr. Bell says. The changes, or oscillations, occur in "decadal phases," which means the cold variations can last for one or several decades. The current cold spell could be an indication of a new decadal phase, one that could mean colder winters for the next 10 or 20 years, but it's too early to tell, Mr. Bell says. The reasons behind the oscillations in the jet stream are just considered "part of the natural weather and climate variability," he adds.

Though El Nino seems to be in hiding right now, it was very noticeable in the fall, when it helped dump heavy rains on parched Texas and Georgia. "El Nino can single-handedly eliminate drought; it can be very beneficial," Mr. Bell says. "The El Nino rains are also a critical component of the desert ecosystem in the Southwest."This El Nino period started early last year and will last from a year to four years. El Nino usually brings with it rain and mild winters. The winter temperature for the entire season so far, which might surprise, is still above average, Mr. Halpert says. The periodic warming of the tropical Pacific Ocean is measured by dozens of buoys placed in the ocean hundreds of miles apart. It's only if the area of warming is large, such as from the international date line (west of Hawaii and east of New Zealand) to South America, that the phenomenon is considered El Nino.The average temperature of the ocean water is about 79 degrees Fahrenheit, but when El Nino occurs, it's about 82.5 degrees Fahrenheit. "At these temperatures, the water is warm enough to support the heavy tropical rains," Mr. Bell says. The warmer ocean creates water vapors, which rise. The vapors eventually form clouds, which contain small drops of water. When these drops become too heavy, they fall to the ground. El Nino alters wind patterns, air pressure, storm tracks and clouds, which affects weather patterns worldwide. In the United States, it's known to release huge amounts of rain. The El Nino phenomenon got its name from fishermen in Ecuador and Peru, who noticed that the ocean suddenly got warmer than normal during the winter months in some years. The phenomenon occurred around Christmastime, and they named it "the Christ Child," or El Nino in Spanish.Scientists can tell when El Nino is about to occur because the water at 200 to 400 feet below the ocean's surface in the tropical Pacific starts to warm. This sign helps forecasters predict when El Nino's more punishing effects, such as heavy rains, will occur.

Another major contributor to the year-to-year variations in winter weather patterns is El Nino's opposite, La Nina, which brings colder and drier weather to the United States. It occurs when tropical Pacific Ocean temperatures drop two to four degrees Fahrenheit below average for an extended period. The latest La Nina occurred in 1998 and lasted through 2000. "The resulting dry weather set the stage for numerous wildfires in the West at that time and contributed to drought across much of the South and East," Mr. Bell says. The natural state of the ocean-atmosphere system is to shift between El Nino and La Nina episodes every few years so that the two never exist at the same time. Sometimes these episodes are very strong, as during the 1982-83 El Nino that caused heavy rains and snow in the mountainous West and wreaked havoc with the California coast, partly because forecasters were unable to predict the strength of the storms. Since then, forecasters have gotten better tools and theories to predict El Nino and La Nina, and when El Nino brought heavy rains to the Southeast in the late '90s, most people were able to prepare.Mr. Bell says the ability to predict weather patterns helps people, on a very basic level, plan their lives. The state of the weather can affect such practical things as whether it's time to fix a leaky roof. Or influence travel plans. "We get people who ask us, 'Do I need to fix my roof right now?' or can they hold off a year? And if El Nino is having a strong year, the answer might be, 'Yes, you need to fix it before the impending heavy winter rains hit,'" Mr. Bell says. "People are empowered by that kind of knowledge that comes from producing highly skillful forecasts often months in advance," he says. For Mr. Byrne, however, it doesn't matter whether El Nino is strong or weak. He'll be out there with Josie but today or tomorrow, if the forecast holds, he might not need those extra layers of clothes. It may no longer be as they referred to it when he was a child in icy Minnesota as "cold as molasses in January."